It is known that electrical equipment and devices, such as high voltage transformers, are usually equipped with bushings, which are suitable to carry current at high potential through a grounded barrier, e.g. a transformer tank or a wall.
Conventional bushings are constituted by an insulator made of ceramic or composite material, which is provided with sheds and is generally hollow. The voltage grading can be obtained with or without a condenser body through which the electrical conductor passes.
An example of a bushing 1 for wall mounting will now be described with reference to FIG. 1 showing the overall structure of the bushing, and FIG. 2 showing a sectional view of the bushing mounted to a wall.
A high voltage conductor 2 extends through the center of a hollow gas filled bushing insulator 4 that forms a housing around the high voltage conductor. A wall flange 6 is provided to connect the housing of the bushing to ground through a wall. The high voltage conductor is provided with a contact 8, 10 in both ends thereof.
A wall 12 is shown in FIG. 2, in which the bushing 1 is mounted by means of the wall flange 6. This figure shows a so-called throat shield or voltage grading shield 14 provided inside the hollow bushing insulator 2 at and around the portion of the bushing going through the wall 12. This shield, which is made of a suitable metal, such as aluminum, accomplishes grading of the electrical field in the bushing and is used instead of a condenser core.
In gas filled high voltage bushings, dust or dirt can become a problem if the dust particles generate partial discharge when hitting a metallic electrode, such as the voltage grading shield.